1. Field of the Invention
The present invention relates to a data transmission method of transmitting high-speed data which are transmitted via a network channel, such as a LAN, to another network channel which is disposed in a moving object or the like and is physically separated from the above-mentioned network channel, by dividing the high-speed data into two or more low-speed data, so that the high-speed data can be used in the other network channel. It also relates to a data transmission apparatus and a data transmission system using the data transmission method.
2. Description of Related Art
In general, when a large amount of such high-speed data as text data and image data is transmitted from a network on the transmit side to another network on the receive side, an attempt to transmit such a large amount of high-speed data while maintaining the transfer rate of the network on the transmit side causes a necessity to provide electronic parts which can carry out high-speed processing with each of the two networks on the transmit side and on the receive side and increases the complexity and cost of the system. Then, in a radio communications system for transmitting such high-speed data using a wireless LAN, a data transmission method of dividing high-speed data into two or more low-speed data and transmitting them is adopted in order to reduce the total cost required to construct the system, for example.
In addition, since those two or more low-speed data into which the original high-speed data is divided and which are transmitted to the other network on the receive side have different delay times dependent upon the conditions of the transmission path and so on, processes of adding a bit (i.e., a framing bit) for delay detection and compensation to each of the two or more low-speed data, determining the differences in delay time among these low-speed data by comparison among the positions of the framing bits respectively added to the two or more low-speed data, and adjusting the delay times are performed on the two or more low-speed data in order to return the positions of all bits of each of the two or more low-speed data which have been shifted due to the delay time differences to the original positions.    [Patent reference 1] Japanese patent application publication (TOKKAI) No. 2002-191073 (see pp. 2 to 4 and FIG. 1)    [Patent reference 2] Japanese patent application publication (TOKKAI) No. 2002-135304 (see pp. 2, and 4 to 6, and FIGS. 1 to 3)    [Patent reference 3] Japanese patent application publication (TOKKAI) No. 2000-92146 (see pp. 2 to 4 and FIGS. 1 to 5)
A problem with the above-mentioned prior art data transmission method is however that since a bit (i.e., a framing bit) for delay detection and compensation is added to each of two or more low-speed data into which original high-speed data is divided, while differences in delay time among the two or more low-speed data can be compensated for, when a source network channel in which transmission of yet-to-be-divided high-speed data is carried out is used at a transfer rate close to the wire rate, a buffer overflow of the data occurs and all data cannot be transmitted from the network channel on the transmit side to another network channel at the receive side. In other words, the wire-rate performance of the source network channel cannot be ensured.
The wire rate of a network channel means a maximum transfer rate of the network channel in consideration of control data and so on. Although it is necessary to increase the processing speed as the amount of control data added to data to be transmitted increases, it is impossible to increase the processing speed and an overflow of the data occurs when the source network channel is being used at a transfer rate close to the wire rate.